1. Field of the Invention
The present invention concerns a method and system for sending data over a transmission link in variable length blocks in asynchronous transfer mode.
2. Description of the Prior Art
Conventional asynchronous transfer mode (ATM) transmission over an ATM transmission link is a digital transmission process in which data is sent in the form of cells. These cells are groups of consecutive bits and include header bits and data bits. The header bits of a cell carry data internal to the ATM transmission network providing the ATM transmission links, for example data identifying the destination of the cell. The data bits convey data sent over the ATM transmission network on behalf of external terminal equipments connected to the network.
One characteristic of ATM transmission is that cells are sent over a transmission link in a continuous uninterrupted sequence. Because of this, when there is no data to be sent an ATM transmission link conveys so-called "empty" cells, that is to say cells containing no user data but containing standardized and readily recognizable service data.
A full description of ATM transmission will be found, for example, in "Asynchronous Transfer Mode" by Martin de Prycker published by Ellis Horwood in 1991 or in the relevant CCITT recommendations.
At present ATM transmission is based on cells which are all the same length and each comprise a header and a data field. Consideration has been given to including in the header control bits, for example bits representing a cyclic redundancy check (CRC) calculation. A calculation of this kind can apply to part only or to all of the header. A typical example of fixed length cells is provided by ATM transmission using 53-byte cells in which each cell comprises a header on five bytes and a data field of 48 bytes.
Consideration has also been given to sending variable length cells in an enhanced transmission process departing from ATM transmission as just defined. Reference may usefully be made to patent applications PCT/EP89/00942 (which corresponds to commonly assigned U.S. application Ser. No. 07/565,310) and EP-90401393.5 (which corresponds to commonly assigned U.S. application Ser. No. 07/776,337). These variable length cells are made up of sub-cells having the same fixed length. The transmission links then carry variable length cells made up of fixed length sub-cells and empty cells. A variable length cell comprises at least one header sub-cell and one or more data sub-cells. An empty cell is the same length as or comprises a sub-cell used for the variable length cells.
Thus in the documents mentioned the sub-cells transmitted are all the same length.
The invention is directed to further enhancement of transmission efficiency as compared with the enhanced transmission method mentioned above.
It relates to a process and a system which differ from ATM transmission so that it has been deemed desirable to make a distinction by using the terms "block" and "sub-block" rather than "cell" and "sub-cell" to denote send data units which are similar to the usual ATM units in many respects but different in others, as will be explained later.
The invention therefore starts from a transmission method using variable length blocks each comprising a header sub-block and one or more data sub-blocks and empty blocks each made up of an empty sub-block, that is three different types of sub-block, in the manner of the method previously described.
It is based on the observation that the conditions determining the optimum length of the sub-blocks of a particular type are not the same as those determining the optimum length of the sub-blocks of another type and that as a consequence of this the optimum lengths of the two types of sub-block are not the same, except where this is coincidental.